tests/basics/gc1: Add test which triggers GC threshold.

tests/basics/gc1.py

@@ -27,3 +27,8 @@ if hasattr(gc, 'threshold'):
     assert(gc.threshold() == 0)
     assert(gc.threshold(-1) is None)
     assert(gc.threshold() == -1)
+
+    # Setting a low threshold should trigger collection at the list alloc
+    gc.threshold(1)
+    [[], []]
+    gc.threshold(-1)